The material dynamics of recording holograms in a holographic medium varies significantly as a function of the number of holograms stored in a single location and also as a function of how many times that location has been read before it has been permanently fixed. Typically, a scheduling technique is used to account for this dynamic effect with the amount of exposure necessary so that all of the recorded holograms within the same location have the same diffraction efficiency, thereby maximizing the storage potential of that location.
In general, a scheduling algorithm is a predetermined exposure sequence based on the material sensitivity and dynamics that dictates how long each hologram should be exposed during the recording process so that every hologram within the same location has equal diffraction efficiency upon readout. The recording process includes simultaneous illumination of a location in a holographic medium by a data beam and a reference beam. A probe beam that is nominally a duplicate of the reference beam that was used for the recording can be used at any time to read the hologram and measure the efficiency of the recording. However, the process of reading the hologram with the probe beam further exposes the material and therefore decreases the sensitivity of the material similarly as in the recording process. Separately using a probe beam in this way therefore decreases the number of holograms that can be written in the corresponding location, which is an undesirable effect.
Reading out a hologram requires that the probe beam be Bragg matched to the hologram. This can lead to a significant amount of alignment difficulty if the probe beam is generated by a source that is separate from the reference beam or if the probe beam has a wavelength different from the reference. Since the diffracted reference-beam has identical phase and amplitude characteristics to the data beam, it cannot be easily separated from the data beam and detected to monitor the holographic writing process unless the data beam is turned off. However, repeatedly turning off the data beam for diagnostic purposes, undesirably interrupts the recording operation.
Therefore, there is a need for monitoring the process of recording a hologram without interfering with the recording process and without creating additional operational difficulties.